See this important circuit update for a fix to the noise floor.

Befaco A*B+C is a dual four-quadrant multiplier with VC offset, a Eurorack utility module with a distinctive design.

This unit does an analog multiplication of two signal (A and B) and add an offset (C).

Signal B has a attenuverter that will multiply by -2 to 2 while Offset has a -1 to 1 attenuverter (unity gain).
B and C inputs are normalized to 5v and 10v so A input can be processed just using the potentiometers.

The output of first block is normalized to second one so second output can act as master, to get (A1*B1+C1) + (A2*B2+C2)

It can act as a dual voltage controlled attenvuerter, dual ring modulator, dual VCA or four channel mixer.

To the left is a photo of my build.  Below is a Befaco photo.  I chose black knobs over the Befaco signature white and red.  I think the black knobs are beautiful against the shiny black panel.

Befaco published this excellent block diagram.

My Build

This is a DIY project, my first for Eurorack where I had to source all of my own parts.  After purchasing a set of PC Boards and front panel from Synthcube, I went looking for the rest of the parts.  The BOM provided on the Befaco site was not especially useful, despite the otherwise good information.  I applaud Befaco for giving the schematic diagram.

Sourcing Parts

The jacks are specified as WQP-PJ301BM.  I found some equivalents on Synthcube.  I had to buy a bag of 50, even though I only needed 16, since I built two of these modules.  It turned out that these jacks are identical to the ones I built the L-1 VC Stereo Mixer with, and that had a benefit in that I knew the four little feet could be cut off (see below).

The pots are special to Befaco and available on Modular Addict.

No part number was given for the knobs Befaco uses.  I purchased black Davies 1900 clone knob with a metal insert from Synthcube.  These work OK, although you tighten the set screw against the splines of the pot shaft.

The remainder of the parts came from Mouser.  Most resistors can be from the 270-VALUE-RC series, which are 1/8 watt.  The 4.7M is Mouser 603-MFR-12FTF52-4M7.  A couple of the needed values were out of stock, especially the 5.36K, so I mistakenly ordered part 71-RN60C5361F.  It was indeed a 5.36K 1/8 watt resistor, but huge in size!  You’ll see how I had to wrangle it in, below.

LED is Mouser 604-WP937EGW.

Power connector is a right-angle, Mouser 710-61201021721.

The standoff I used is an 11 mm with M3 thread, Mouser 855-R30-1001102.  But I discovered that a 12 mm length would be needed, so I added a washer.  You would want 855-R30-1001202.  You’ll need one standard M3 panel screw, which you probably have lots of already.

The AD633 balanced modulator chip is Mouser 584-AD633ANZ.  A bit pricey, but it’s the most expensive part.  The TL072CN and TL074CN are standard.

Headers for connecting the two PC boards are Mouser 710-61301021821 and 855-M20-9980545.

Capacitors

100nf:  581-SA105E104M.  I use them for all 0.1uf bypassing duties.

220pf ceramic: 80-C410C221J1G.

10uf electrolytic: 647-USR1E100MDD.  The important feature of this part is its small size.

Construction

Befaco’s assembly instructions are not too bad, but I can add to them.  First of all, contrary to how they tell you to assemble the front panel, with the pots not fully seated to the board, I have a better way!  If you cut off the small tabs on the pots AND if you cut off the four small ‘feet’ on each jack, allowing the jack to sit flush, you will find that the heights of the pots and jacks are in perfect alignment.

My steps

1. Put the two sets of headers together and temporarily fasten the two boards together with the 12 mm standoff and the headers in place.  The male header goes on the main (rear) board.  Spread a 1 millimeter space between the mated headers, so that the bottoms of the headers are flush with the PC boards.  Now solder the headers.  Separate the boards.
2. Fit the right-angle power connector in place and solder.
3. Wash the PC boards, if you are using organic flux, and let dry.
4. Assemble all parts on the main board.  IC sockets are optional.  Solder and wash.
5. Assemble all but the jacks, pots, and LEDs on the front board.  IC socket optional. Solder and wash.
6. Fit the jacks, pots and LEDs into the front board, but do not solder.  Place the panel over the jacks and pots and add their nuts, finger tight only.
7. Using no-clean solder, solder the jacks, pots, and LEDS into place.  Do the LEDs last, so you can put them at just the right height.
8. Gently tighten up the panel nuts for the pots and jacks.
9. If you used IC sockets, install the chips into them.
10. Connect the two boards together, securing with the 3M screw into the standoff.
11. Affix your knobs of choice.

I had to cut off the end of one leg of a jack that protruded into the power connector.  Everything else cleared just fine.

Pictures

I was frustrated that I could not find any rear photos of the A*B+C.  So I took plenty.

See that weird resistor jutting up, just behind the standoff screw?  That is R107.  It is supposed to go on the back of the board, but the one I had was too big to fit behind the power connector.  Fortunately I was able to fit it on the top, behind the panel.  The other similar resistor, R102, was easier to fit on the back.  See below to the left.